Dense Molecular Clumps with Large Blue Asymmetries: Evidence for Collapse

An analysis of the Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey has produced a sample of 27 candidate dense molecular clumps with large collapse motions, as revealed by large ``blue’’ asymmetrical line profiles of the optically thick \hcop, line. %with respect to the optically thin \nthp, line. New, more sensitive molecular line observations of this sample, conducted with the Mopra 22-m telescope, confirm the blue asymmetries in the \hcop, line profiles, with large, positive values of the asymmetry parameter $A$ (${\bar{A}}_{HC{O}^{+}}=0.69\pm 0.01$), and positive, but smaller asymmetries in the \hcn, and \hnc, lines: (${\bar{A}}_{HCN}=0.35\pm 0.01$ and ${\bar{A}}_{HNC}=0.28\pm 0.01$), as expected for a less optically thick tracer in collapsing clumps. The small, positive mean asymmetry parameters for \cch, and \htcop, ${\bar{A}}_{C_{2}H}=0.15\pm 0.02$ and ${\bar{A}}_{H^{13}C{O}^{+}}=0.18\pm 0.03$, likely indicate slightly optically thick emission for at least some clumps. The hyperfine ratios for \nthp, are in their optically thin, LTE, values, but for \hcn\ they are not; the $F=1\to 1$ hyperfine line shows abnormally weak intensities. A simple two-component model shows that self-absorption of the background $F=1\to 1$ hyperfine line by the main $F=2\to 1$ hyperfine line of a cold, foreground, redshifted cloud can reproduce the observed \hcn, hyperfine intensities and match the \hcn, and \hcop, line profiles. All of these results are consistent with self-absorption of the optically thick lines on the red side of the profile, as expected for collapsing clumps. A simple two-cloud model suggests that this sample represents dense clumps with extreme collapse velocities, $V_{inf}\sim 2.4$ \kms.